This invention relates to the art of differential amplifiers and more particularly to such an amplifier employing JFET transistors operating in the ohmic region.
The differential amplifier will be described herein as being in the input stage of an electrometer; although, it is to be appreciated that the invention may be applied to other applications requiring an amplifier exhibiting high input impedance, for measuring voltage, and low input bias current, or leakage current for measuring current.
Differential amplifiers are frequently employed in applications for measuring voltage and current, and in such applications are used in the input stage of an electrometer. Electrometers are known in the art, and typically serve to measure primarily DC voltage and current, although many also include extensions of these parameters, such as resistance or charge measurement. For measuring voltage, a distinguishing feature of the instrument is a very high input resistance, typically on the order of 10.sup.13 to 10.sup.15 ohms. This is important when measuring voltages from sources having high series resistance which would be loaded excessively by normal digital voltmeters (DVM). As an example, measurement of electrochemical EMF or pH requires high input resistance.
When an electrometer is employed for measuring current, it should exhibit very low input bias current (leakage current) since the current resolution of an electrometer picoammeter may well be in the range of fA (10.sup.-15 A) or aA (10.sup.-18 A). A typical application of an electrometer picoammeter is the measurement of a very high resistance on the order of 10.sup.12 to 10.sup.16 ohms by impressing a voltage across the resistance in series with the picoammeter.
In order to achieve an input stage for an electrometer having the characteristics of high input impedance and low leakage, it has heretofore been common practice to employ an input stage comprised of MOSFET transistors. The MOSFET transistors are acting as an input stage to a differential amplifier, sometimes referred to as an operational amplifer or OP-AMP. An example of such a MOSFET OP-AMP input stage for an electrometer is found in the U.S. Pat. No. to Shah, 3,654,468. Such an input stage has exhibited the desirable high input impedance and low leakage (or input bias) current required of an electrometer. However, the high input impedance achieved with a MOSFET transistor is primarily dependent on the insulating properties of a thin silicon dioxide layer, sometimes referred to as the gate insulation layer. While such a circuit exhibits the desired low input bias current characteristic, it does exhibit poor overload characteristics unless suitable protection circuitry is provided. Thus, input voltage transients on the order of 30 volts or more may cause breakdown or puncture of the gate insulation layer. Additionally, such MOSFETs frequently require careful offset, temperature compensation, common mode rejection, noise and bias current selection which may be batch sensitive with low yields.
It has been known in the past to employ JFETS instead of MOSFETs as the input stage for an electrometer. Such JFETs are connected in a differential amplifier and have exhibited improved voltage characteristics as compared with those of MOSFETs, but typically have higher input bias current by a factor of 10, for example. Thus, a MOSFET input stage is known to exhibit a low input bias current on the order of less than 5 fA (5.times.10.sup.-15 A). On the other hand, a known JFET input stage, while exhibiting excellent voltage characteristics, may have input bias current which is on the order of 60 fA. It is important that the electrometer have an input bias current which is substantially less than the input current to be measured. Such a large input bias current of known JFETs severely limits the applications to which an electrometer employing such a JFET input stage may be applied.